1. Field of the Invention
The present invention relates to a method of removing an organic polymer film, particularly, to a simple and convenient method of completely removing a polyimide film as an alignment film in the manufacture of a color filter substrate used in a liquid crystal display device. It is absolutely necessary to remove completely the polyimide film for the rework of a color filter substrate. And also, the present invention relates to a method of removing an organic polymer film on an array substrate.
2. Description of the Related Art
The manufacturing process of a cell of a liquid crystal display device includes the step of forming an organic polymer film serving to orient liquid crystal molecules followed by forming a color filter structure or an array structure on a substrate. The organic polymer film includes a film of, for example, polyimide resin (PI) and polyvinyl alcohol (PVA), and is formed on the entire surface of the substrate by a printing method or a spin coating method. Then, the film is baked, followed by applying an aligning process such as rubbing to the baked film so as to form an alignment film. However, where a defect is included in the organic polymer film thus formed, the entire substrate was discarded as a defective substrate. Since the substrate including a color filter structure or an array structure is prepared by employing a manufacturing technology of a very high level in order to achieve a high density and a high precision in a display device, the discarding of the entire substrate, which is defective, leads to an increased manufacturing cost.
In order to improve the situation, it is attempted to once remove the defective film of the organic polymer, followed by newly forming a satisfactory film. As the technique to remove the organic polymer film, some methods now in use for removing a photoresist film in the manufacturing process of a semiconductor device and the like, are applicable. For example, a wet etching is one of the well-known method for removing an organic polymer film in which the organic polymer film is dissolved in an appropriate solvent. In the case of a polyimide film, -butyrolactone, N-methyl pyrrolidone (NMP), etc. are used as the solvent. Even in the case of using such a solvent, it is impossible to remove completely the residual polyimide, with the result that a polyimide film having a thickness of about 10 to 100 is left unremoved on the substrate. Also known is a dry etching method such as a plasma etching method or an ozone ashing method. It may be possible to achieve the complete removal of the residual polyimide film by employing these dry etching methods. However, the dry etching method requires a vacuum apparatus and an exclusive ozone generator. In other words, a relatively costly and complex apparatus and manufacturing step is required in the dry etching method, compared with the other method. Such being the situation, the dry etching method has not yet been put to practical use. The dry etching method gives rise to an additional difficulty. Specifically, it is necessary to rotate or rock the substrate, making it difficult to deal with substrates of various sizes, particularly, a large substrate sized at 600xc3x97720 mm.
Japanese Patent Disclosure (Kokai) No. 6-202111 discloses a method of removing a polyimide liquid crystal alignment film. It is disclosed that the film is exposed to ultraviolet radiation having a wavelength of 230 to 300 nm, followed by dipping the film in a polar solvent or an alkaline solvent. However, it has been found that the ultraviolet radiation having a wavelength of 230 to 300 nm is transmitted through the underlying transparent conductive film of ITO (Indium-Tin-Oxide), though the transmission is only several percent, so as to do damage to the pigment contained in the color filter positioned below the ITO film. For example, discoloring is brought about in the pigment.
Also, it has been found that it is impossible to remove completely the polyimide film by only exposure to ultraviolet radiation having a wavelength of 230 to 300 nm.
As described above, it is unavoidable for an organic polymer film on a substrate for a cell of a liquid crystal display device to bear a defect caused by a defective printing or by a damage done during the aligning process such as rubbing. However, it was impossible to remove completely the organic polymer film for reworking the substrate. It is of high importance to develop a cheap and simple method for removing completely the organic polymer film such as a polyimide film attached to the substrate in order to improve the manufacturing yield and to reduce the manufacturing cost of the liquid crystal display device.
An object of the present invention is to provide a simple and convenient method of completely removing an organic polymer film on a substrate.
Another object of the present invention is to provide a simple and convenient method of completely removing a polyimide film used as an alignment film but being defective in the manufacture of a color filter substrate and an array substrate for a liquid crystal display device. The complete removal of the defective polyimide film is absolutely necessary for the reworking of the substrate. It follows that an additional object of the present invention is to provide a method which permits improving the manufacturing yield of a cell of a liquid crystal display device.
Still another object of the present invention is to provide a simple and convenient apparatus for completely removing an organic polymer film on a substrate.
According to the present invention, an organic polymer film, which is to be removed from a substrate surface, is exposed to ultraviolet radiation having a wavelength of 180 nm or less so as to decompose and remove the organic polymer film. This particular removing method is useful for removing an alignment film comprising an organic polymer such as polyimide to rework the substrate.
The principle of the removing method is shown in FIG. 1. To be more specific, the film is considered to be removed by a breaking function, that is, each bond of organic polymer in the film is broken by the energy of the irradiated ultraviolet radiation, and by an oxidizing function performed by oxygen within the air atmosphere, which is excited by the energy of the ultraviolet radiation. In general, the coupling energy (eV/molecule) between adjacent atoms contained in an organic molecule is mainly: Cxe2x80x94N bond (3.02); Cxe2x80x94C bond (3.60); Cxe2x80x94H bond (4.29); and Cxe2x95x90C bond (6.29). The energy (E) of the ultraviolet radiation is determined by:
E=hc/excex
where h represents the Planck""s constant (6.626xc3x9710xe2x88x9234 [Jxc2x7sec]), c represents the speed of light (2.998xc3x97108 [m/sec]), e represents the energy of a single electron (1.602xc3x9710xe2x88x9219 [J/eV]), and xcex represents the wavelength of the light [m]. The energy of the ultraviolet radiation emitted from a conventional low pressure mercury lamp having an output wavelength of 254 nm is 4.88 eV, which is smaller than the coupling energy of the Cxe2x95x90C bond noted above. In other words, it is impossible for the ultraviolet radiation having a wavelength of 254 nm to break the Cxe2x95x90C bond. It follows that the pyromellitic acid portion and the condensed ring portion of the polyimide structure given below are not decomposed but remain on the substrate: 
On the other hand, ultraviolet radiation having a wavelength of 180 nm is capable of imparting an energy of 6.89 eV that is large enough to break the Cxe2x95x90C bond. Therefore, the interatomic bonds included in almost all the organic polymer or molecule can be broken and thus the organic polymer or molecule can be decomposed.
Also, the oxygen molecules within the atmosphere generate excited oxygen atoms O* when exposed to ultraviolet radiation as shown below: 
The excited oxygen atom O* readily reacts with each of the atoms and atomic groups broken by irradiation of the ultraviolet radiation and convert atoms of C, N and H into molecules of CO2, NOx and H2O. These molecules are gaseous and, thus, can be moved from the substrate into the atmosphere. It follows that these gaseous molecules can be removed easily by discharging the atmosphere from within the chamber. Alternatively, the excited oxygen atom is considered to attack directly the organic polymer so as to oxidize the particular material and, thus, to generate the similar gaseous oxide molecules.
However, the high energy of the ultraviolet radiation having a wavelength shorter than 180 nm may do damage to the substrate. For example, it is possible for the particular ultraviolet radiation to decompose even the useful material contained in the structure below the film of the organic polymer. For example, if the pigment portion is exposed directly to ultraviolet radiation in the color filter substrate included in a liquid crystal display device, the pigment portion suffers from a damage such as discoloring. It should be noted in this connection that a transparent electrode pattern made of ITO or IZO (Indium-Zinc-Oxide) is formed on the surface of the pigment portion in the ordinary color filter substrate. It has been found, however, that ultraviolet radiation having a wavelength not longer than 180 nm is scarcely transmitted through the particular transparent electrode pattern, with the result that the particular electrode pattern acts as a protective film of the pigment portion. Thus the method of the present invention makes it possible to remove easily the polyimide alignment film on a substrate including a color filter structure.
On the other hand, the typical array structure as shown in FIG. 4(a) for a liquid crystal display device is adversely effected by the directly exposure to ultraviolet radiation, transistor characteristics of the semiconductor layer are shifted, and electrification will be occurred in the gate insulating layer. In particular, TFT characteristics are changed such as that the gate threshold voltage is decreased and that the drain off current is increased. As the result, maintaining the charge at the TFT is leaked to make a white spot on the display and to lead a poor display quality. However, an array structure having a polymer film on the array (it is called as PFA structure or PFA substrate, hereafter) as shown in FIG. 4(b) which has a ITO or IZO film on the surface of the TFT element may avoid the damage by exposure to the ultraviolet radiation, because the lower semiconductor layer can be protected by the ITO or IZO film. This PFA structure is developed in order to increase the aperture ratio by the structure having a transparent electrically conductive film which acts a role of a gate line, on the TFT structure through an organic insulating layer. Therefore, the removing of the polyimide alignment film on the substrate having PFA structure is easily achieved by the present invention.
The thickness of the ITO or IZO film formed on these substrates is approximately 1500 angstrom to maintain the high transparency in the visible light but it is sufficient to shield the ultraviolet radiation having a wavelength of 180 nm or less.
Furthermore, in light of the above principle, it is easily understood that the object to be decomposed and removed is not only polymer but also any organic molecules. An example of such organic molecules is a contamination attached on the substrates during manufacturing process. Thus, the removing method and the apparatus can be applied to removing a residue of resist composition, a residue after typical cleaning and a surface contamination in the manufacturing process for not only LCD but also microelectronics or semiconductor device.